[Python-ideas] Programming recommendations (PEP 8) and boolean values

[Michael Foord]

Hey all,

True and False are singletons, so if you want to check precisely for True
and False then an identity check seems appropriate.

However the "Programming Recommendations" section of PEP 8 has this to say
on the topic:

Don't compare boolean values to True or False using ==.

Yes:   if greeting:
No:    if greeting == True:
Worse: if greeting is True:

http://www.python.org/dev/peps/pep-0008/#programming-recommendations

It seems to me that there is an important distinction between testing that
an object is either the boolean True or False and merely checking the
"truthiness" of an object. Many a bug has been caused by an empty container
object (or some other falsey object) falling into an "if not value" clause
that was actually meant to check for the presence of False or None.

Why does PEP 8 recommend not testing "boolean values" (which to me implies
"values you expect to be a bool") using equality or identity, and more
specifically why does it say that using an identity check is worse than an
equality check?

As this is Python-ideas and not python-list, my specific suggestion is to
modify the wording of that section - or just removing it altogether as I
don't think it can be adequately clarified without using lots of words.

All the best,

Michael Foord

-- 

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May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html
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